Various methods for extracorporeal blood treatment or cleaning are used for the purpose of removing substances usually eliminated with urine and for the purpose of withdrawing fluid. In the case of hemodialysis, the patient's blood is cleaned outside the body in a dialyzer. The dialyzer has a blood chamber and a dialyzing fluid chamber, which are separated by a semipermeable membrane. During the treatment, the patient's blood flows through the blood chamber. In order to clean the blood effectively from substances usually eliminated with urine, fresh dialyzing fluid continuously flows through the dialyzing fluid chamber.
Whereas the transport of the smaller molecular-weight substances through the membrane of the dialyzer is essentially determined by the concentration differences (diffusion) between the dialyzing fluid and the blood in the case of hemodialysis (HD), substances dissolved in the plasma water, in particular higher molecular-weight substances, are effectively removed by a high fluidic flow (convection) through the membrane of the dialyzer in the case of hemofiltration (HF). In hemofiltration, the dialyzer acts as a filter, which is therefore referred to in the following as a dialyzer. Hemodiafiltration (HDF) is a combination of the two methods.
In the case of hemo(dia)filtration (HDF), a part of the serum withdrawn via the membrane of the dialyzer is replaced by a sterile substitution fluid, which is supplied to the extracorporeal blood circuit upstream and/or downstream of the dialyzer. The supply of substitution fluid upstream of the dialyzer is referred to as pre-dilution and the supply downstream of the dialyzer as post-dilution.
In an extracorporeal blood treatment, the ultrafiltration rate (UF rate) is of interest, which is a measure of the amount of fluid withdrawn from the patient within a time interval. The ultrafiltration rate is dependent on transmembrane pressure TMP in the extracorporeal blood treatment, the ultrafiltration rate increasing with increasing transmembrane pressure.
Transmembrane pressure TMP is defined as the pressure difference between the mean blood-side pressure and the mean dialysate-side pressure on the dialyzer. In principle, four pressure measurements are required for an exact determination of the transmembrane pressure, the pressure being measured at the inlet and outlet of the blood chamber and inlet and outlet of the dialyzing fluid chamber of the dialyzer. For this purpose, a pressure sensor is required in each case at the blood-side inlet and outlet and at the dialysate-side inlet and outlet of the dialyzer.
In practice, however, the measurement of the transmembrane pressure by means of four pressure sensors proves to be relatively expensive. For reasons of technical simplification, therefore, the determination of the transmembrane pressure by means of four pressure sensors is generally refrained from in practice.
For the determination of the transmembrane pressure, it is known to determine the pressure solely by means of two pressure sensors, whereof one pressure sensor is disposed on the blood side and the other pressure sensor on the dialysate side. For reasons of handling and cost, it is proposed, for example in the article by H. D. Polaschegg “Methods and history of ultrafiltration control in haemodialysis” (Aktuelle Nephrologie, vol. 1/1985, page 135 and following), to restrict the measurement to the venous backflow pressure and the pressure at the dialyzing fluid outlet.
Apart from the determination of the transmembrane pressure by means of two pressure sensors, the determination of the membrane pressure by means of three pressure sensors is also known. For the determination of the transmembrane pressure, European patent application publication EP 0 212 127, for example, proposes measuring the pressure in the dialyzing fluid supply line and discharge line and the pressure in the blood return line, in particular the drip chamber disposed in the blood return line, and calculating the transmembrane pressure on the basis of the measured pressures. The calculated transmembrane pressure is compared with a predetermined setpoint value for the mean transmembrane pressure, in order to adjust the dialyzing fluid pump disposed in the dialyzing fluid discharge line. The suction pump on the dialyzing fluid side is regulated in such a way that the transmembrane pressure in the dialyzer is kept at the setpoint value.
In practice, the determination of the transmembrane pressure on the basis of only two or three pressure measurements, whereof one pressure measurement takes place on the blood side and the other measurement on the dialysate side in each case, has been considered to be sufficiently accurate. The inventors have found, however, that under certain treatment conditions limiting factors have to be placed on the determination of the transmembrane pressure with a high degree of accuracy.
A goal of the present invention is to provide a method for determining the transmembrane pressure in an extracorporeal blood treatment, which on the one hand requires only a relatively small technical outlay for the measurement and on the other hand guarantees a high degree of accuracy under all treatment conditions.
Moreover, it is a goal of the present invention to provide a device for determining the transmembrane pressure for an extracorporeal blood treatment apparatus, which permits a determination of the transmembrane pressure with a high degree of accuracy with less than four pressure sensors under all treatment conditions.
Further goals of the present invention are to provide a method for extracorporeal blood treatment and an extracorporeal blood treatment apparatus, wherein the determination of the transmembrane pressure takes place with a high degree of accuracy with a relatively low technical outlay.